Introduction
Welcome to the free notes for Class 9 Science Chapter 2 Cell: The Building Block of Life Notes. In this chapter, students learn about the discovery of cells, cell theory, plant and animal cells, cell organelles, and their functions. These notes cover the key concepts for quick revision and understanding.
Based on the Latest NCERT Textbook
These notes are prepared according to the latest NCERT Exploration: Textbook of Science for Grade 9 and cover all important concepts in an easy-to-understand format.
Table of Contents
What is a Cell?
A cell is the basic structural and functional unit of life. Every living organism is made up of one or more cells, and all life processes take place within cells.
Discovery of the Cell
- The cell was first observed by Robert Hooke in 1665 while examining a thin slice of cork under a microscope.
- Antonie van Leeuwenhoek was the first to observe living cells.
Cell Theory
The cell theory states:
- All living organisms are made up of cells.
- The cell is the basic structural and functional unit of life.
- New cells arise from pre-existing cells.
Types of Cells
Unicellular Organisms
Made up of a single cell.
Examples:
- Amoeba
- Paramecium
- Bacteria
Multicellular Organisms
Made up of many cells.
Examples:
- Human beings
- Mango tree
- Dog
Cell Organelles
Nucleus
The nucleus is a membrane-bound organelle found in most eukaryotic cells. It is often called the control centre of the cell because it regulates all the activities of the cell, such as growth, metabolism, protein synthesis, and cell division.
The nucleus contains the cell’s genetic material (DNA) in the form of chromosomes. These chromosomes carry hereditary information and pass genetic traits from parents to offspring.
Structure of the Nucleus
The nucleus consists of:
- Nuclear Membrane: A double-layered membrane that protects the nucleus and controls the movement of substances in and out.
- Nucleoplasm: A jelly-like fluid inside the nucleus that supports its internal structures.
- Nucleolus: A dense, round structure responsible for producing ribosomes.
- Chromatin/Chromosomes: Thread-like structures containing DNA and genes.
Functions of the Nucleus
- Controls all cellular activities.
- Stores hereditary information (DNA).
- Regulates cell growth and reproduction.
- Directs protein synthesis.
- Helps in cell division.
Mitochondria
Mitochondria are small, double membrane-bound organelles present in the cytoplasm of most eukaryotic cells. They are known as the “Powerhouse of the Cell” because they produce the energy required for various cellular activities.
Mitochondria generate energy in the form of ATP (Adenosine Triphosphate) through the process of cellular respiration. This energy is used by the cell for growth, movement, repair, and other life processes.
Structure of Mitochondria
- Outer Membrane: Smooth membrane that surrounds the mitochondrion.
- Inner Membrane: Folded into finger-like projections called cristae, which increase the surface area for energy production.
- Matrix: The inner fluid-filled region containing enzymes, mitochondrial DNA, and ribosomes.
Functions of Mitochondria
- Produce energy (ATP) through cellular respiration.
- Supply energy for all cellular activities.
- Help in the metabolism of carbohydrates and fats.
- Contain their own DNA and ribosomes, allowing them to produce some of their own proteins.
Chloroplast
Chloroplast is a green, double membrane-bound cell organelle found only in plant cells and some algae. It contains a green pigment called chlorophyll, which absorbs sunlight and helps plants prepare their own food through the process of photosynthesis.
Since chloroplasts help plants make food, they are known as the “Kitchen of the Cell.”
Structure of Chloroplast
- Outer Membrane: Protects the chloroplast.
- Inner Membrane: Surrounds the inner contents of the chloroplast.
- Stroma: Jelly-like fluid containing enzymes, DNA, and ribosomes.
- Grana: Stacks of disc-shaped structures called thylakoids, which contain chlorophyll.
Functions of Chloroplast
- Carries out photosynthesis.
- Contains chlorophyll to absorb sunlight.
- Produces glucose (food) for the plant.
- Releases oxygen during photosynthesis.
- Stores starch produced during photosynthesis.
Golgi Body (Golgi Apparatus)
The Golgi Body, also known as the Golgi Apparatus or Golgi Complex, is a membrane-bound cell organelle found in both plant and animal cells. It was discovered by the Italian scientist Camillo Golgi in 1898.
The Golgi Body acts as the packaging and distribution centre of the cell. It modifies, packages, stores, and transports proteins and other substances to different parts of the cell or outside the cell.
Structure of the Golgi Body
- It consists of a stack of flattened, membrane-bound sacs called cisternae.
- Small vesicles are present around the Golgi Body to transport materials.
- It is located near the endoplasmic reticulum (ER) and the nucleus.
Functions of the Golgi Body
- Modifies proteins and lipids received from the endoplasmic reticulum.
- Packages proteins and other substances into vesicles.
- Transports materials within the cell and outside the cell.
- Forms lysosomes.
- Helps in the secretion of hormones, enzymes, and other substances.
Difference Between Plant Cell and Animal Cell
| Plant Cell | Animal Cell |
|---|---|
| Cell wall present | Cell wall absent |
| Chloroplast present | Chloroplast absent |
| Large central vacuole | Small vacuoles |
| Fixed shape | Irregular shape |
Important Terms
- Cell
- Cell Membrane
- Cell Wall
- Cytoplasm
- Nucleus
- Organelle
- Chloroplast
- Mitochondria
- Vacuole
Chapter Summary
✔ Cell is the basic unit of life.
✔ Every organism is made up of cells.
✔ The nucleus controls cell activities.
✔ Mitochondria are known as the powerhouse of the cell.
✔ Chloroplasts help in photosynthesis.
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FAQ
What is a cell?
A cell is the smallest structural and functional unit of life.
Who discovered the cell?
Robert Hooke discovered the cell in 1665 while observing cork under a microscope.
What is the function of the nucleus?
The nucleus controls all the activities of the cell and contains genetic material.
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